The present invention relates to motion control systems and, more specifically, to linear motor systems that provide independent control of multiple moving elements.
The application of such devices to a wide variety of processes (e.g. packaging, assembly automation, processes involving use of machine tools, etc.) provides the advantage of increasing both the speed and flexibility of the process. Since linear motors employ a moving magnetic field to directly drive the moving element, they eliminate the gear heads, shafts, keys, sprockets, chains and belts often used with traditional rotary motors. This reduction of mechanical complexity provides both reduced cost and increased speed capability by virtue of reduced inertia, compliance, damping, friction and wear. This technology also provides greater flexibility than rotary motors by allowing each individual moving element and its associated tool or payload to be independently servocontrolled along its entire path.
Typical systems use “incremental absolute” position sensors which can determine absolute position only after a mover that moves along the track is jogged past a known reference point. Thereafter, these systems count incremental pulses (or digitize sine/cosine signals to create these pulses) to count up or down from this reference point. If power is lost or the pulse counting is disrupted due to a crash, jam, etc., then these prior art systems have to be re-homed and commutation alignment performed again. This results in lost production time and, oftentimes, lost product.
Traditional linear motors suffer from a variety of limitations. For example, U.S. Pat. No. 5,965,963 discloses a linear motor with a plurality of independently movable elements. Each movable element has a track of switching sensors associated with it to connect proximal armature coils to a power source associated with each movable element. In addition, a separate track of encoder sensors is provided for each movable element to measure its position. This configuration can limit the number of movable elements and the length of the track since the mechanical complexity and electrical reliability of the system becomes impractical with a large number of sensor and switch tracks and elements. Additionally, the position sensing system disclosed is an incremental type, with the limitations described above.
PCT Publication WO 00/75603 discloses a machine that utilizes two or more magnetostrictive sensors disposed along a path for position sensing. The sensors are linked into a single “virtual” sensor by a digital signal processor, which outputs a position signal that can be read by a motion controller. The magnetostrictive sensors measure position by timing acoustic waves along a wave guide. The typical acoustic speed for these sensors is 2800 m/s, which is very slow compared to the speed of an electron or photon. For example, a typical system might require a resolution 0.1 mm at a speed of 1000 mm/s for a path that is 3 meters long. A large number of magnetostrictive sensors are needed, which have been prohibitively expensive.
U.S. Pat. No. 6,191,507 discloses a modular conveyor system comprised of interconnected track sections. Each section has a plurality of individually controlled coils that provide independent control of multiple pallets that travel along the track. This system employs a plurality of linear encoder readers spaced at fixed positions along the track and linear encoder strips mounted on the pallet to sense the position of each pallet. Each section has a controller for trajectory generation, position compensation and current control.
The position sensing system disclosed, however, is an incremental absolute type. Since it uses incremental encoders, it can provide an absolute position signal only after performing a homing and commutation alignment procedure for each movable element at power up. This requires moving each element a certain distance to find the zero reference position and the magnetic pole positions. After abnormal events e.g. crashes, jams, etc. incremental position pulses are often lost and the system must be re-homed. Additionally, this system utilizes a series of encoders that overlap to maintain the absolute position reading of each pallet. This overlap design requires additional complexity to hand-off the control from reader to reader.
Third, the permanent-magnet attractive force is not balanced resulting in reduced bearing life as compared to a design with a balanced magnet load.